1. Field of the Invention
The invention relates to a method for imaging a pattern arranged in an object plane of a projection lens onto an image plane of the projection lens, a projection lens for carrying out the method, and a method for fabricating such a projection lens.
2. Description of the Related Art
Illumination methods and projection lenses of the types are employed on projection illumination systems, such as wafer scanners or wafer steppers, that are used for fabricating semiconductor devices or other types of microelectronic devices and serve to project reduced images of patterns present on photomasks or reticles, which shall hereinafter be generically referred to as xe2x80x9cmasksxe2x80x9d or xe2x80x9creticles,xe2x80x9d at ultrahigh resolution onto an object that has been coated with a layer of photosensitive material.
Improving the spatial resolutions of the projected images of masks having increasingly finer patterns will necessitate both increasing the numerical aperture (NA) of the image end of the projection lens involved and employing light having shorter wavelengths, preferably ultraviolet light having wavelengths less than about 260 nm.
There are few materials, in particular, synthetic quartz glass and crystalline fluorides, such as calcium fluoride, that are sufficiently transparent at such short wavelengths available for fabricating the optical components involved. However, the materials suffer from photoelastic effects, i.e., may affect the mutually orthogonally polarized components of the field vectors of transmitted light differently due to stress birefringence.
Since the Abbxc3xa9 numbers of those materials that are available all lie rather close to one another, it is difficult to configure systems consisting entirely of refractive components that have been sufficiently well-corrected for chromatic aberrations. Catadioptric systems that combine refracting and reflecting components, i.e., in particular, lenses and mirrors, are thus predominantly favored for configuring high-resolution projection lenses. Such systems frequently employ deflecting mirrors that are used at large angles of incidence and serve to deflect light traveling between the object plane and image plane of the projection lenses to one or more concave mirrors or to deflect light reflected by same back to same. In order that the mirrors will have high reflectivities, they will usually have reflective coatings, which will normally be multilayer reflective coatings. The curved surfaces, some of which may be sharply curved, of lenses will also usually be coated in order to reduce reflections, where light transiting some of same will also have large angles of incidence on same. However, employing dielectric coatings on optical components that involve large angles of incidence may affect transmitted light in various ways that will depend upon its polarization.
In the case of catadioptric systems of the aforementioned type, it has been found that the projected images of lines present on the patterns on the masks will frequently exhibit contrast variations that will depend upon the orientations of the lines. The variations in image contrast with orientation, which are also termed xe2x80x9chorizontal-vertical variationsxe2x80x9d (H-V variations), will be reflected in discernible variations in the imaged widths of the lines on photoresists, where their imaged widths will depend upon their respective orientations.
Various means for avoiding such orientation-dependent contrast variations have been proposed. European Pat. No. EP 964 282 A2 concerns itself with the problem that catadioptric projection systems that employ deflecting mirrors introduce a preferred polarization direction for light transiting same that is attributable to their multilayer-coated deflecting mirrors having differing reflectivities for s-polarized and p-polarized light, which will cause light that is unpolarized at their reticle plane to become partially polarized when it reached their image plane, which, in turn, will cause their imaging characteristics to vary with orientation. According to the proposal presented there, this effect may be counteracted by creating a lead polarization by creating partially polarized light having a prescribed residual polarization within the illumination system involved that will then be compensated for by its projection lens such that light exiting the latter will be unpolarized.
A catadioptric projection lens having a polarization beamsplitter that is also supposed to minimize orientation-dependent contrast variations is known from European Pat. No. EP 0 602 923 B1, which corresponds to U.S. Pat. No. 5,715,084. The projection lens, which is used with linearly polarized light, has a means for altering the state of polarization of light transiting same that transforms incident linearly polarized light into circularly polarized light, which, in terms of the intensities of its orthogonally polarized field components, is equivalent to unpolarized light, situated between the projection lens"" beamsplitter cube and its image plane, which is intended to provide that image contrast will be independent of the orientations of patterns appearing on masks. A corresponding proposal was also made under European Pat. No. EP 0 608 572, which corresponds to U.S. Pat. No. 5,537,260.
However, it has been found that contrast variations among patterns that have differing orientations on masks may still occur, particularly in the case of catadioptric projection lenses operated with large aperture and having at least one deflecting mirror, in spite of those measures to counteract same that have been described above.
One object of the invention is to provide an illumination method, particularly one that may be employed in microlithography, a projection lens that will be suitable for use in conjunction with same, and a method for fabricating the projection lens that will allow avoiding the disadvantages of the prior art. It is another object to provide a projection lens that allows imaging patterns such that their images will exhibit virtually no contrast variations due to differences in their orientations.
As a solution to these and other objects, this invention, according to one formulation, provides a method for imaging a pattern arranged in an object plane of a projection lens onto an image plane of the projection lens comprising the following steps:
illuminating the pattern with light for creating a light beam with a first light ray having a first polarization direction and a second light ray having a second polarization direction that differs from the first polarization direction;
transmitting the light beam through the projection lens, wherein the light rays of the light beam are incident on the optical components of the projection lens at differing angles of incidence and wherein a difference in the lengths of a first optical path traversed by the first light ray and a second optical path traversed by the second light ray occurs within at least one region of the image plane due to the optical configuration employed; and
compensating for the difference in optical path length by intentionally altering at least one of the first optical path and the second optical path such that the difference in the length of the optical path traversed by the first light ray and that traversed by the second light ray occurring within the image plane is largely independent of their angles of incidence.
Beneficial embodiments of the invention are stated in the dependent claims. The wording appearing in all of the claims is herewith made a part of the contents of this description.
In the case of the method according to the invention, the projection lens will be transited by a light beam whose electric-field vector either has a first component and a second component orthogonal thereto or an electric-field vector that may be decomposed into the orthogonal components, where the ratio of the components of the electric-field vector will determine the state of polarization of the light beam.
Rays of the light beam are incident on the optical components of the projection lens at various angles of incidence. The optical components constitute an optical system that will yield a difference in the length of a first optical path traversed by the first rays of the light beam, which have a first polarization direction, and the length of a second optical path traversed by the second rays of the light beam, which have a second polarization direction differing from the first polarization direction, over at least one region of the projection lens"" image plane. According to the invention, the difference in the lengths of the optical paths will be compensated for by altering the length of the first optical path and/or the length of the second optical path such that any difference between the length of the first optical path traversed by the first ray and the length of the second optical path traversed by the second ray will be largely independent of their respective angles of incidence on the optical components.
A desired difference in the lengths of the optical paths traversed by the light rays having differing polarization directions that will depend upon the locations of the traversals and the traversal angles involved will thus be created by incorporating at least one means for optically correcting for the difference in the lengths of the optical paths that has been especially adapted to suit the projection lens, where the desired difference in the lengths of the optical paths will be such that it will exactly compensate for any undesired difference in the lengths of the optical paths caused by the other optical components of the projection lens. The difference in the lengths of the optical paths may be described in terms of a differential wavefront that will be defined at every point on the illuminated field and may vary from point to point thereon. The compensated difference in said optical path lengths cannot be further compensated employing conventional means that affect light having differing polarization directions equally and are thus incapable of compensating for any difference in their respective optical path lengths. However, means for optically correcting for same that are in accordance with the invention can be employed for compensating for those imaging errors due to stress birefringence and employment of dielectric coatings mentioned at the outset hereof and will thus improve the performance of said projection lens. The means for optically correcting for the difference in optical path length are alternatively denoted as optical correction means throughout this application. The proposed correction of the relative-phase surfaces of mutually orthogonal polarization directions will not alter the field amplitudes and thus will not adversely affect the brightnesses of partial images. On the contrary, the locations and shapes of said partial images will be altered such that all of same will occur at the same locations and have the same shapes. The effect may be achieved by arranging that the compensation will be such that light rays having mutually orthogonal polarization directions will have traversed optical paths of virtually equal length upon arrival at the image plane of the projection lens. A corresponding effect will also occur if a difference in the lengths of the optical paths that will remain constant over the range of the angles of incidence involved is introduced.
The invention is based on the recognition that the polarization-sensitive effects, in particular, such due to employment of materials exhibiting stress birefringence or employing dielectric coatings on optical components that involve large angles of incidence, will cause the wavefronts for s-polarized and p-polarized light to differ at the image plane, where the wavefronts for s-polarized and p-polarized light may be tilted with respect to one another, astigmatically distorted, defocused, and/or otherwise distorted due to various departures from design specifications and fabrication tolerances that may occur from projection lens to projection lens. Since light having differing polarization directions interfere independently, pairs of partial images that may be vertically or laterally displaced with respect to one another or displaced with respect to one another along the optical axis of the projection lens, where the displacements along the optical axis of the projection lens may also vary with the orientations of those patterns being imaged, will be created. The invention eliminates the displacements of partial images having a specific polarization in order to avoid variations in image contrast due to variations in the orientations of the patterns.
Imaging errors may also vary over the optically utilized field. The dependence upon location will lead to variations in their critical dimension (CD) over the field, which will cause the images of lines having equal widths to have widths that will vary with the latter""s locations within the field, which, in turn, may further degrade the qualities of the finely patterned devices fabricated. However, the invention is also capable of eliminating these types of imaging errors.
One type of projection lens has a first, catadioptric section situated between its object plane and its image plane and a concave mirror, a beam-deflecting device, and a second, dioptric section that follows the concave mirror in the optical train. The beam-deflecting device, which is configured in the form of a reflecting prism, has a first, coated, deflecting mirror that deflects radiation coming from the projection lens"" object plane to the concave mirror and a second, coated, deflecting mirror that deflects radiation reflected by the concave mirror to the second, dioptric section. The basic layout of the projection lens may correspond to that depicted in European Patent Application No. EP-A-0 989 434, which corresponds to U.S. patent application Ser. No. 09/364,382, where the concave mirror is housed in a side arm inclined at an oblique angle with respect to the principal optical axis of the projection lens involved.
In the case of reduction lenses of the type, the angles of incidence on the first, coated, deflecting mirror are larger than those on the second, coated, deflecting mirror. The inventor has determined that the predominant effect of the coatings employed on the deflecting mirrors is a tilting of the wavefronts of s-polarized and p-polarized light toward the scanning direction (y-direction). The partial images created by s-polarized and p-polarized light may be laterally separated by several nanometers due to the tilting alone, which will cause the partial images of lines oriented along the scanning direction to be effectively superimposed on one another, while the partial images of lines orthogonal to the scanning direction will be displaced, yielding pairs of parallel lines, which will smear out and broaden the images of the lines, which is why a preferred other embodiment of the highly preferable type of projection lens provides that the compensation for the difference in the optical path lengths of light rays having differing polarizations will introduce a gradient, preferably a linear gradient, in the difference in the optical path lengths transverse to the optical axis of the projection lens that will allow compensating for the tilting of the wavefronts. A suitable means for optically correcting for the tilting of the wavefronts might have at least one wedge-shaped retarding element fabricated from a birefringent material, i.e., from a material whose optical properties vary with polarization direction, where the retarding element may be fabricated from, e.g., magnesium fluoride or strained calcium fluoride. Suitably orienting a retarding element of the type in the vicinity of a pupillary plane of the projection lens, i.e. in or near that pupillary plane, will accurately compensate for any tilting of the differential wavefront due to the former""s wedge shape. Adding a second wedge fabricated from an isotropic material whose optical properties are independent of polarization will then effectively transform the combination of the wedge-shaped retarding element and the second wedge into a plane-parallel plate that will have only a slight, evenly balanced, effect on light rays having either polarization direction.
As an alternative thereto, or in addition thereto, the means for optically correcting for the tilting of the wavefronts might have at least one retarding element fabricated from a birefringent material having a gradient in the difference in its refractive indices for light having the differing polarization directions transverse to the optical axis of the projection lens.
The optical correction means, i.e. the means for optically correcting for the difference in the optical path lengths of light having differing polarization directions that introduces a desired (compensating) difference in same that will depend upon the locations and angles at which the light transits the optical components of the projection lens may, in principle, be arranged anywhere within the projection lens. The means of optical correction might either consist of discrete optical components or be implemented by applying suitable coatings to existing optical components of the projection illumination system. Imaging errors that are uniformly distributed over its entire field should preferably be compensated for in the vicinity of a pupil of the projection lens, i.e. in or near that pupil plane. On the other hand, any gradients in polarization-sensitive wavefront errors should preferably be compensated for in the vicinity of, or near, a field plane of the projection lens, which will introduce a phase shift in the field components that will vary over its field plane.
The invention, in another formulation, also relates to a method for fabricating a projection lens containing optical components that introduce a difference in the optical path lengths traversed by light having differing polarization directions in the manner described at the outset hereof, where the method comprises the following steps:
configuring the projecting lens using the optical components;
transmitting light beams through the projection lens, wherein the light beams contain a first light ray and a second light ray that is orthogonally polarized with respect to the first light ray;
determining wavefronts for light rays transmitted by the projection lens, wherein a first wavefront for the first light ray and a second wavefront for the second light ray are determined in accordance with their polarizations, and the wavefronts are employed for determining differential wavefronts;
creating at least one optical correction means in accordance with the differential wavefronts, wherein the optical correction means are configured for compensating for the differences in the lengths of the optical paths such that the differential wavefronts will be largely independent of their angles of incidence on the optical components due to the compensation for the differences in the lengths of the optical paths when the optical correction means is installed in the projection lens; and
installing that optical correction means in the projection lens.
In particular, the correction may be carried out such that the differential wavefront virtually vanishes.
For example, computational simulations based on measured material parameters, such as the characteristics of coatings, may serve as a basis for fabricating a means for the optical correction, in which case, the layout of the projection lens and ray tracing may be carried out entirely with computer assistance or virtually carried out. Determination of the differential wavefront and designing suitable means for optically correcting for same may also be carried out entirely with computer assistance or virtually carried out.
However, the differential wavefront may also be empirically determined using an actual, fully assembled, projection lens. For example, transmitted wavefronts or ray paths may be measured independently for each polarization direction using a polarizer and a polarization analyzer. The resultant measurements on an actual, fully assembled, projection lens may be employed in fabricating a compensating element that has been tailored to suit same that may then be installed therein.
Prospective compensating effects of the means of optical correction according to the invention are not confined to the compensation for wavefront tilting that has have been described above in terms of examples. For example, concentric differential-wavefront aberrations, such as those that may occur when polarized light transits antireflection coatings on lenses at large angles of incidence, are also correctable, either alternatively, or in addition thereto. Employing more complexly shaped and/or strained means of optical correction and/or more complexly configured gradients in their birefringent effects will allow compensating for arbitrary distortions of the differential wavefront of s-polarized and p-polarized light.